There are many instances where loading effects on induction motors vary, in some instances, varying from a full load condition to an essentially no-load condition. In other instances, devices connected to a motor product a variable coupling which may vary from a state of the imposition of a substantial load on the motor, through a zero load state, and through one or more drive states wherein the device applies discrete torques back on the motor.
One example of such a multi-state operation occurs with lift pumps, such as used in pumping oil from oil wells. During the upstroke, or portion of the pumping stroke during which oil is being lifted, the motor applies a torque to the pump; and during a portion of the return, or downstroke, of the pump, torque is applied by the pump back to the motor. Depending upon the force of this downstroke, the motor will, to a degree, be caused to speed up and may exceed the basic synchronization speed (line frequency divided by pairs of poled of the motor gives sync speed in resolutions per second) of the motor. When this occurs, there will be two effects. One, the motor commences to generate power and to supply it back to the power line to which it is connected; and two, coordinately, the motor reflects a braking force back on the pump. Both these effects increase with applied torque generally up to approximately the horsepower rating of the motor. The effective speed range for power generation varies with different motors from approximately synchronous speed to 101% to 104% of sync speed.
It is obvious that the generating effect described is most desirable, as it reduces the average power drawn by the motor. This reduction is manifested by either the reduction in rate of operation of a power meter, or, where no other power is being drawn by the user of the motor, by an actual reversal of the power meter during the power generation state. Significant to the present invention, the efficiency of power generation by an induction motor simply directly connected to a power line varies directly as a function of the driving force. Thus, if the driving force is equal to or near the horsepower rating of the motor, then efficiency and power generation is at a maximum for that motor. However, if the driving force is less than this, which is often the case, then the efficiency of power generation declines. This efficiency relationship is essentially the same for an induction motor operating in a motor mode, as its efficiency suffers substantially when it is operated in either an unloaded or a lightly loaded mode.
The applicant has previously addressed both the motor mode and generating mode efficiency problems, with the motor mode being dealt with in his Pat. Nos. 4,052,648 and 4,266,177 and in referenced pending application Ser. No. 297,524, and the generating mode being the subject of the applicant's referenced pending application Ser. No. 243,683.
U.S. Pat. Nos. 4,052,648 and 4,266,177 and pending application Ser. No. 297,524 disclose control circuitry wherein power loss normally experienced in unloaded or lightly loaded induction motors are substantially reduced by varying the power input to the motor as a direct function of the detected power factor of operation of the motor.
The applicant's pending application Ser. No. 243,683 discloses a system for improving the efficiency of an induction motor-type generator when it is less than fully driven. In that system, the induction motor, as a generator, is coupled to a commercial power line to which it delivers its output through a triac, which is controlled to have a relatively small, fixed firing angle.
It is the object of this invention to provide a control system which is adapted to optimumly couple an induction motor to a power line to enable it to operate with increased efficiency both during times when it is providing rotary power to a load and during times when a rotary driving force is applied to it.